1. Field of the Invention
The present invention relates to an initialization method for phase change type optical disc and particularly, to capable of improving recording characteristic and erase ratio of an optical disc.
2. Description of the Background Art
Generally, a phase change type optical disc is a kind of optical recording media for repeatedly recording and reproduction information by enabling recording and erasing the information using a reversible transformation of a recording layer.
The phase change type optical disc melts a local area of the recording layer by irradiating the area with concentrated laser beam and transforms an amorphous mark in a crystalline matrix by rapidly cooling the area using a disc structure rapidly designing diffusion of heat. Accordingly, the information is recorded and an amorphous mark is transited to a crystallite by heating the portion in recording information thereafter with a lower power than in the former recording operation, thus to erase the recorded information.
The matrix portion of such phase change type optical disc is transited into a crystallite before recording the information since the step of transiting the crystallite into amorphous is defined as a first step for recording the information. The step of transiting the initial amorphous into crystallite is defined as initialization.
FIG. 1 is a schematic cross-sectional view showing each layer composing a conventional phase change type optical disc and FIG. 2 is a partially enlarged view showing a recording layer of the optical disc in accordance with the conventional initialization method for the phase change type optical disc.
As shown in FIG. 1, the phase change type optical disc 10 includes a substrate 11 which is transparent, has a disk shape and is formed with a member having a predetermined stiffness, a lower dielectric layer 12 formed on the upper surface of the substrate 11 along the thickness direction, a recording layer 13 formed on the upper surface of the lower dielectric layer 12, an upper dielectric layer 14 formed on the upper surface of the recording layer 13 and a reflection layer 15 and protection layer 16 formed on the upper surface of the upper dielectric layer 14.
The substrate 11 is made from glass, plastic and the like which is transparent and have a predetermined stiffness and polycarbonate having an excellent ability of injection molding, for preventing reduction of signal-to-noise ratio (SNR) by briefrengence in case of incident of laser beam is widely used.
The recording layer 13 can be reversely changed between the amorphous state and crystalline state and intermetallic compound of ternary system alloy represented by Ge—Sb—Te is widely used. Recently, Ag—In—B—Te designed to raise erase ratio and signal recording quality at low linear velocity is given more attention.
At the upper side and lower side of the recording layer 13, a dielectric layer for maintaining optical and thermal characteristics is formed and ZnS—SiO2 group thin film is used as the lower dielectric layer 12 and upper dielectric layer 14. As the reflection layer 15, thin films of Al alloy, Ag, Au and the like are used to have a proper cooling speed after irradiating with laser beam increasing light reflection amount.
The protection layer 16 formed on the upper surface of the reflection layer 15 is formed by hardening the ultraviolet hardening resin using a ultraviolet lamp after coating the resin to have a predetermined thickness using a spin coater and the like.
On the other hand, the phase change type optical disc 10 having such multilayer structure is initialized to change the amorphous state to crystalline state by irradiating the recording layer 13 with laser beam before recording.
And, when the phase change type optical disc 10 is initialized, the optical signal, that is, reflectivity is changed as the state of the disc is changed to crystalline state. The reflectivity can be increased or decreased according to the thin film designing value of the optical disc but generally the reflectivity is increased.
Namely, in case reflectivity of the phase change type disc 10 is too high after initializing, reflectivity after initialzing is 50% or higher, absorption of energy is decreased and accordingly the recording characteristic is deteriorated with the conventional recording laser beam.
In case reflectivity is too low, that is, in case the reflectivity after initializing is 10% or lower, there can occur problems in tracking of laser beam or degradation of the recording layer 13 can be increased by absorbing excessive amount of energy.
Therefore, taking these problems in consideration, the phase change type optical disc 10 is formed to have 20 to 40% of reflectivity after the conventional initialization. Also, the disc is initialized by increasing the power of the initializing laser beam higher than a predetermined value.
On the other hand, the phase change type optical disc is changed to the crystalline state after initializing and the crystallite has a different grain size distribution according to the reflectivity. The grain size must be uniform with a proper size. When the size is too large or small, the recording characteristic and repeating recording characteristic are deteriorated.
Therefore, it is hard to maintain the proper initializing level in case of initializing as conventionally. Namely, in case the value of reflectivity by the excessive initialization is near from the saturation value which can be occurred in the given thin film structure, a crystal grain 18 having an excessive size is formed in the recording layer 13 as shown in FIG. 2 and accordingly, recording is not performed well or recorded mark is not erased well even if recording is done with a high power, thus to deteriorate the erase ratio.
Also, if the initializing level is too low, recording operation was not performed well since crystallinity of crystallite forming a matrix is low.